At least two collagen chains alpha 1 and alpha 2 are synthesized by human lung in vitro. Over a wide age range in the adult lung approximately 4 percent of the amino acids incorporated into protein per hour are incorporated into collagen. These studies are being used to quantitate the degree of the fibrotic process in the interstitial disorders, to develop an in vitro system for testing drug efficacy in the fibrotic diseases and to examine the interaction of serum proteins on the rates of proteolysis of lung connective tissue components synthesized in vitro.